1. Field of the Invention
The present invention relates to metal-coated substrates, and in particular to a metal-coated transparent substrate sheet in which the metal is silver, gold or copper or an alloy of one or more thereof.
Such coated substrates find application as, for example, window glazing for buildings or vehicles, in laminated structures and as optical filters.
Panels used as window glazing in buildings desirably have a low solar factor (FS, defined below) so that the interior of the building is not overheated by solar radiation, while also having an adequate level of luminous transmittance (TL, defined below) so as to provide sufficient internal light. These somewhat conflicting requirements can also be expressed as a need for the panel to display good selectivity, i.e. a high ratio of luminous transmittance to solar factor (further defined below). It is also desirable that the glazing panels present both a neutral aspect in reflection, otherwise expressed as a low colour purity, and a low level of reflection but these are properties which tend to be difficult to achieve in combination with high selectivity.
2. Description of the Related Art
It is known to improve the selectivity of a metallic coating on glazing by increasing the thickness of the metallic layer(s). However this has the disadvantage of imparting to the glazing a purple colour in reflection, which tends to be regarded as aesthetically unattractive. It has therefore been proposed to overcome the disadvantage by use of an oxide layer beneath the metal, the said oxide layer having a higher refractive index than other oxide layers in the coating, whereby interference effects are created between the respective oxide layers which reduce the undesired coloration. French patent specification 2719036 teaches the use as such an underlayer of an oxide selected from niobium oxide and tantalum oxide. Such a high refractive index underlayer does not however favour the achievement of a low luminous reflectance (RL), and consequently of a high selectivity, of the coated substrate. Our earlier GB patent specifications 2229737 and 2229738 describe a substrate of a glazing material carrying a multi-layer coating which comprises a reflective layer of silver sandwiched between a transparent undercoat and a transparent overcoat. The undercoat comprises at least one layer of a metal oxide over which is deposited a layer of zinc oxide up to 15 nm thick. The overcoat comprises a layer of an oxide of a sacrificial metal such as aluminium, bismuth, stainless steel, tin or titanium or of a mixture thereof.